Aerosolized bronchodilator therapy, wherein a measured amount of medication-containing aerosol is inhaled by an asthmatic patient, is considered the mainstay of therapy for asthma. A long-standing problem of this type of treatment is the proper timing of the release of the bronchodilator medication with respect to the onset of inspiration. If an asthma patient receives most of the aerosolized medication at the onset of inspiration, the medication can be carried deeply into the lungs and thus constitute more effective therapy. In contrast, if the aerosolized medication is dispensed too early or too late with respect to the onset of inspiration, the medication will be sprayed ineffectively into the mouth, or may be inhaled only shallowly, so that the medication is expelled without ever reaching the lungs when the patient exhales. This problem becomes even more apparent when considered in the context of a severe asthma attack, for as bronchoconstriction worsens, the patient's ability to inspire becomes progressively more limited. With such a severely limited airflow, the asthma patient must receive the bronchodilator medication at the very onset of inspiration, or else the limited amount of air able to reach the patient's lungs will not carry the essential medication.
Other inhalers known to the art have not successfully confronted this problem, and, as will be seen, allow a patient to spray the medicine into his mouth out of synchronization with inhalation. For example, U.S. Pat. No. 3,456,646 discloses an aerosol inhaler which comprises a housing, a air passage within the housing, a vane disposed in one end of the air passage, a mouthpiece disposed at the other end of the air passage, and an aerosol container being reciprocally received in said housing, disposed so as to inject a metered amount of medication-containing aerosol into the air passage. In this apparatus, a mechanical linkage activated by the vane prevents the aerosol container from discharging its medication into the air passage so long as the vane is in its normal "closed" position. The user places his mouth on the mouthpiece and inhales, creating a pressure differential which causes the vane to pivot, displacing the mechanism linkage and thereby permitting the aerosol container to reciprocate into the housing and to inject its measured amount of medication into the air passage.
U.S. Pat. No. 3,732,864 discloses another aerosol inhaler which discharges its medicine during the patient's inhalation, the inhalation serving to release a gate which normally blocks actuation of the device. This apparatus is similar to that disclosed in the '646 patent, but with the mechanical linkage eliminated and the gate disposed directly underneath the aerosol container. When the user inhales on the mouthpiece, the gate, which normally restrains the aerosol container in its fully extended attitude, rotates out of the way, thereby permitting the container to reciprocate into the housing and dispense its measured amount of medication.
Both of these prior art inhalers require a predetermined minimum airflow during inhalation before medication can be discharged into the housing for inspiration by the user. Additionally, neither of these prior art inhalers synchronizes the release of the medication with the onset of inhalation, and the inhalers thus are often ineffective in delivering the medicine deep into the lungs. Moreover, since these devices require a certain minimal vacuum and airflow in order to operate the valve mechanism, a severely distressed patient may not be able to generate sufficient suction to operate the inhaler, thereby precluding the very therapy so desperately needed. In addition, if a user is uncertain whether he has succeeded in properly inhaling the medication into his lungs, he may be disposed to discharge the device more than once, which could lead to a dangerous overdosage of the medication.